He tought appeassement would make US stop. He has even accepted US sanctions on Nkorea hoping US will stop harassing china. Appeassement for the sake of appeassement is not a strategy. Overall china foreign policy is weak and don't seem to have a very clear objective. Over reacting so quickly to low level challenge but not reacting to true challenge.
As if North Koreans getting nukes is a good news to China.
Can you even pinkie swear that North Korea is not geopolitically or otherwise a threat to China? North Korean nukes can also fly westwards...ya'know.
North Korean Juche is all about the Kim family. They'll do anything for preserving the Juche. And China wants a denuclearized North Korea as much as South Korea or Japan.
Take the map of Bohai Sea. (It is similar to the Gulf of Mexico, strategically or otherwise). Notice the country that borders it besides China - That is North Korea.
It is against China's national interest to have any other country (foe or friend) pose hurdles for unchallenged utilization of that sea for China. Ideally, China ought to have the North Korean shore of Bohai Sea secured so that the entire Bay is up for China's free unchallenged liberal use. A prosperous North Korea that could raise its voice in International forum will one day clash with China over its use of Bohai Sea.
Not withstanding these very specific geography question ,
A nuclear North Korea is a bigger threat to China than a nuclear South Korea. The unstable politics of North Korea is the reason. As of right now, South Korea remains a better neighbour of China than North Korea.